Y-chromosome effects on Drosophila geotaxis interact with genetic or cytoplasmic background

Previously, all of the major fruit fly, Drosophila melanogaster, chromosomes (I, II and III) have been shown to be associated with geotaxis, but the Y chromosome has not. Using two methods (back-crossing and chromosome substitution), Y chromosomes from lines that have evolved stable, extreme expressions of geotaxis were placed into different genetic and cytoplasmic backgrounds to test the resulting males for geotaxis. The results of the back-crossing do not support the interpretation of Y-chromosome effects on geotaxis. These tests do not have sufficient statistical power, however, to detect small genetic effects. In the chromosome substitution experiment, the geotaxis-line Y chromosomes were placed into high- and low-selected lines, Canton-S and Champaign wild-type backgrounds. The results of the chromosome substitution experiment provide evidence for a Y-chromosome effect on geotaxis in selected geotaxis lines, but not in wild-type stock, backgrounds. These results suggest that the Y chromosome has a small effect on geotaxis, whose detection depends on genetic and/or cytoplasmic background. The implications of these results are discussed for behaviour-genetic analysis of D. melanogaster and for issues of statistical power in detecting small genetic effects.